Oops, I'm a bit late on this one. Please consider my post as these books have immensely
impacted my understanding of the world and I couldn't recommend them more strongly.

COSMOS; Carl Sagan (my favorite book of all time, absolutely wonderful, the perfect non-fiction book)
The Demon-Haunted World; Carl Sagan (a hefty lesson in skeptical inquiry and baloney detection)
Pale Blue Dot; Carl Sagan (although the majority of the book is tailored to those with an interest in astronomy, the first 5 chapters are heavily atheistic)
Shadows of Forgotten Ancestors; Carl Sagan / Ann Druyan (a beautifully written book on how evolution shaped our various human behaviors)
The Dragons of Eden; Carl Sagan (Sagan's main book on evolution, I haven't finished it yet, also written beautifully and poetically)

Death by Black Hole; Neil deGrasse Tyson (a humorous / modern version of Carl Sagan's COSMOS)

Life & Time; Isaac Asimov (Asimov makes the technical side of evolution / human development incredibly simple to understand)
The Genetic Code; Isaac Asimov (an incredibly easy to understand introduction to biology / chemistry and the operations of DNA)
Of Matters Great & Small; Isaac Asimov (random tidbits about reality and how these discoveries were made through the scientific method)

Big Bang - Simon Sing: This book highlights the history of science and the acceptance of the Big Bang theory over others like the Steady State Model and shows the evidence that caused each new idea to be recognized.

The end of faith : religion, terror, and the future of reason - Sam Harris: I haven't read it, but I have heard good reviews for it.